Kent police's blanket use of stop-and-search powers on thousands of environmental activists at the Kingsnorth demonstration was "disproportionate and counterproductive", according to an official review into the force's handling of protests released today.

A total of 8,218 searches were carried out on protesters at the week-long demonstration last August against the energy company E.ON's proposed coal-fired power-station, after orders from senior commanders were misinterpreted "as an instruction to search everyone".

Although "huge amounts of property were seized" during the climate camp protests, only 2,000 stop-and-search forms – fewer than 25% – were legible. The report said this raised questions about the competence of police officers and their understanding of the law.

Most protesters were stopped under section 1 of the Police and Criminal Evidence Act (Pace), which requires officers to have reasonable suspicion that an individual is carrying prohibited weapons or articles that could be used for criminal damage.

David Howarth, the Liberal Democrat justice spokesman, echoed the report's findings when he said: "This is yet another example of the disproportionate use of stop and search, and shows how, even on the report's own narrow terms, this tactic is totally counterproductive."

The scale of the stop-and-search operation came to light in two inquiries by the National Policing Improvement Agency (NPIA) into Kent police's £5.3m operation, the largest of its kind in the UK last year. More than 1,400 officers were drafted in from 24 forces to assist with the operation, codenamed Oasis, on the Hoo peninsula.

The Kent force has come under sustained criticism for its management of the demonstration, after allegations of brutality by officers who had covered their badge numbers and concern that police used "psychological operations", including playing loud music at night to deprive activists of sleep.

The force was also forced to apologise after an investigation by the Guardian established its officers had placed journalists covering the demonstration under surveillance.

The reports – an initial debrief by the NPIA and a broader review conducted on its behalf by South Yorkshire police – found the Kingsnorth operation was "in the main successful" because it had stopped protesters getting on to the site and ensured there was "no interruption to power supply". However, many of the concerns put forward by demonstrators appear to be substantiated in its findings.

The reviews paint a picture of widespread breakdown in communication, with police officers from visiting forces given hardly any explanation about why they had been deployed by Kent. They found officers on the ground were under-trained, did not understand their powers, lacked knowledge of basic public order terminology and were given outdated intelligence.

The reports were most critical of the stop-and-search policy, which saw all protesters made to line up in airport-style checkpoints to be searched going to and from the camp. Commanders, the review reveals, initially told officers that "personal grounds must be justified and no blanket power approach is to be taken" when searching under section 1 of Pace. But they were then told "that the camp is illegal and the intention of the camp is to commit damage, hence the grounds for searching attendees to the camp is made", which resulted in almost every activist being searched multiple times.

The reports said this resulted in a "vicious cycle", "moving non-activists closer to resistance and violence on account of tactics they saw hard to accept as justified by the police. With this developing crowd dynamic of hostility, intelligence then presented a worsening picture, which provided more grounds to search camp attendees."

A list of more than 2,000 possessions taken from protesters, released under the Freedom of Information Act, revealed that, in a supposed attempt to prevent activists causing injury or taking a nearby river, officers took packets of balloons, tents, a clown's outfit, camping equipment, cycle helmets and bike locks, bin bags, party poppers, leaflets and soap.

The NPIA debrief was particularly critical of the failure of officers to write legibly. "The fact that so many forms were submitted in such poor quality also raises questions regarding the effectiveness of supervision and the overall knowledge of policing powers, which was felt to be lacking."

Kent police's chief constable, Michael Fuller, welcomed the "numerous areas of good practice", while accepting that there were some lessons to learn. "While many of the recommendations made to us have already been adopted in the intervening 12 months, there is still work to be done either within Kent or in conjunction with other forces or agencies."

The review recommended the Kent force should have worked more closely with E.ON. However, Howarth said it was not the police's job to take the side of companies during legitimate protest.

"It is quite wrong to suggest that the police should have worked more closely from the start with the energy company. The police are not a private security firm. Their job is to act in the public interest, and the public interest includes the right to protest."

The report also said the government should consider introducing new legislation to allow a senior officer to authorise stop and search where "widespread acts of criminal damage was likely". But Howarth dismissed the recommendation. "It is bizarre to suggest that the right response to excessive use of stop and search should be a change in the law to make stop and search more widely available."